The present invention generally involves well logging instruments, and more specifically discloses embodiments directed to combination instruments which perform such functions as measuring formation density and wellbore diameter in addition to running radioactive logs such as a neutron log.
Previous tool designs accomplished these functions using an elongated body having a widely extendable decentralizer arm and drag pad, which arm also comprised part of the caliper instrument. On the back side of the body was the heavy density pad which preferably was kept close to the tool body for accuracy of the logging function. It was preferable to make this pad slightly extendable from the body, for instance one-fourth to three-fourths of an inch extension.
In one type of prior art device, this slight extension of the so-called "fixed pad" containing the logging signal transmitter and receiver, was achieved prior to lowering the tool into the well, and the fixed pad was actually pinned into place in its extended position. Thus, the pad became known as the fixed pad because it remained in the extended position at all times when in the wellbore.
The disadvantage in the fixed pad design is that it undergoes considerable wear and deterioation moving up and down the borehole at speeds of up to 500 feet per minute. This would also shake and dislodge the electrical components in the pad, causing malfunction there.
One solution to the problem has been to incorporate a complex hydraulic-mechanical system into the tool to provide for extension of the density pad at a selective position in the borehole. Examples of this are found in U.S. Pat. Nos. 3,356,146 and 3,254,221. This complex hydro-mechanical system requires extensive modification of the tool configuration to incorporate the needed pad extension components. In addition, the density pad extension is accompanied by extension of a hydraulic cylinder into the wellbore, and control of the amount of extension is lacking at the top of the assembly.
The present invention overcomes these disadvantages by providing a mechanical linkage assembly which is not overly complex yet which provides a predetermined extension of the density pad at the desired time in the wellbore. The invention does not require extension modification of the tool configuration to achieve the desired pad extension.